The present invention relates to a semiconductor device for generating at least two different internal source voltages or, in particular, to a dynamic random access memory (DRAM) having a boosting circuit capable of being tested even when the output of the boosting circuit drops temporarily under an overloaded state.
In recent years, a semiconductor device, or especially a DRAM, has used, as a source voltage, the power supplied from an external source dropped by an internal power supply for reducing the current consumption of the DRAM. The internal power supply will hereinafter be called the VII power supply and the source voltage generated therein the VII. Also, in the DRAM, a 100% charge is accumulated in a memory cell configured of a NMOS transistor, and in order to improve the operating characteristic and stability, a word line supplying a gate voltage of a cell transistor is set to a potential not lower than an external source voltage. For this purpose, the boosting circuit is provided. The power supplied from the boosting circuit will hereinafter be called the VPP power supply, and the voltage generated therein and higher than the external source voltage will be called the VPP. In the case where a device has an internal power circuit for generating two different positive source voltages, circuits coexist which are operated by different power supplies, and a signal may be input from one circuit to the other. In such a case, the potential levels of the signals fail to coincide and therefore voltage conversion is necessary.
The boosting circuit for generating the VPP power has a complicated configuration and is low in conversion efficiency. Also, a boosting circuit having a large output capacity is bulky in circuit size. In view of this, as few circuits using the VPP power supply as possible are employed, and the VII power supply is used as far as possible for those parts where such power is usable. The capacity of the VPP power supply is set as required. Normally, the voltage of the VPP power supply does not drop so much. When testing a product, however, an extremely large load capacity may be driven. In such a case, a large current flows and a voltage drop occurs. This phenomenon may develop as a transient phenomenon in the normal operation as well as at the time of testing a product. Once such a voltage drop occurs, the transistor in the conversion circuit fails to turn off completely and a short-circuit current flows so that the signal fails to drop to the ground level completely. This short-circuit current enhances the voltage drop and the decline of the output level of the VPP power supply itself. In other words, once a short-circuit current begins to flow, even though a transient phenomenon, the voltage drop caused by the short-circuit current enhances the short-circuit current, and the normal state cannot be restored.
Methods have been conceived to obviate this problem. One of them consists of increasing the capacity of the VPP power supply in order to prevent the decline of the level of the VPP power supply. Another method consists of avoiding the voltage conversion from the signal of the VPP power supply to the signal of the VII power supply. Still another method is using a complicated circuit for voltage conversion, if required, from the signal of the VPP power supply to the signal of the VII power supply. These methods, however, require a considerably redundant circuit design of a semiconductor device and cannot be recommended from the viewpoint of device performance and high integration.